Preamp low pass filter
 

I have just acquired a slightly beat-up Cambridge A75 preamp (thanks Rob!).
Aside from some tidying up, this will form part of my intended system
upgrade to a bi-amped setup with a low-pass filter on the bass end. I'm
attracted to fitting a second stereo output stage and filter to the preamp -
that would let me try various amps for driving the bass speakers without
having to deal with incorporating the filter into each. Although I can
operate a soldering iron and a multimeter, I'm not terribly clued up on
silicon electronics...

I'm sure there are chips that can take an input from the PCB and drive a
second pair of outputs to the same spec as the existing outputs - but what
ones would be suitable? Those low noise mosfet 741-thingies?

Is there a chip that I can use to construct a filter which doesn't introduce
the sort of phase problems that a speaker-side crossover would? How much
scope is there for being able to tailer the slope of the filter, and make
the slope selectable? Can a subsonic shelf be incorporated? It would be good
to be able to switch the filter off and have the second output provide full
bandwidth.

I'd be grateful for suggestions of other ways to approach the filtering,
suitable chips, maybe some web sites that have circuit / schematics.



--
Wally
www.artbywally.com
www.wally.myby.co.uk/music

Preamp low pass filter
 

Wally wrote:

I have just acquired a slightly beat-up Cambridge A75 preamp (thanks
Rob!). Aside from some tidying up, this will form part of my intended
system upgrade to a bi-amped setup with a low-pass filter on the bass end.
I'm attracted to fitting a second stereo output stage and filter to the
preamp - that would let me try various amps for driving the bass speakers
without having to deal with incorporating the filter into each. Although I
can operate a soldering iron and a multimeter, I'm not terribly clued up
on silicon electronics...

I'm sure there are chips that can take an input from the PCB and drive a
second pair of outputs to the same spec as the existing outputs - but what
ones would be suitable? Those low noise mosfet 741-thingies?

NE5532 would be my choice.

Ian

Is there a chip that I can use to construct a filter which doesn't
introduce the sort of phase problems that a speaker-side crossover would?

Very difficult to design a filter that changes amplitude with frequency
without affecting phase. Laws of physics type stuff.

How much scope is there for being able to tailer the slope of the filter,
and make the slope selectable?

No problem provided steps of about 6dB/octave are acceptable.

Can a subsonic shelf be incorporated?

No problem, in fact hard not to do this.

It
would be good to be able to switch the filter off and have the second
output provide full bandwidth.

Bypass is easy too.

Ian

Preamp low pass filter
 

Ian Bell wrote:

Very difficult to design a filter that changes amplitude with frequency
without affecting phase. Laws of physics type stuff.


Easy enough to do with a digital signal.

A quick demo would be to low-pass a file with Goldwave or similar,
reverse the file, filter it again and reverse it again.

In the analogue world:

You can keep the low-pass and high-pass outputs in phase
with a 4th-order filter. I got a cheap Soundtech active crossover,
which does this, on ebay. It's easy enough to build one though.

There's some interesting stuff at Siegfried Linkwitz's site:
http://www.linkwitzlab.com

--
Roger.

Preamp low pass filter
 

In message , Wally
writes
I have just acquired a slightly beat-up Cambridge A75 preamp (thanks Rob!).
Aside from some tidying up, this will form part of my intended system
upgrade to a bi-amped setup with a low-pass filter on the bass end. I'm
attracted to fitting a second stereo output stage and filter to the preamp -
that would let me try various amps for driving the bass speakers without
having to deal with incorporating the filter into each. Although I can
operate a soldering iron and a multimeter, I'm not terribly clued up on
silicon electronics...

I'm sure there are chips that can take an input from the PCB and drive a
second pair of outputs to the same spec as the existing outputs - but what
ones would be suitable? Those low noise mosfet 741-thingies?

Is there a chip that I can use to construct a filter which doesn't introduce
the sort of phase problems that a speaker-side crossover would? How much
scope is there for being able to tailer the slope of the filter, and make
the slope selectable? Can a subsonic shelf be incorporated? It would be good
to be able to switch the filter off and have the second output provide full
bandwidth.

I'd be grateful for suggestions of other ways to approach the filtering,
suitable chips, maybe some web sites that have circuit / schematics.




If you want an easy way to design and simulate low/high/band-pass
filters of either Butterworth. Bessel or Chebychev with a conventional
op-amp architecture with first to eighth order response, then go to the
Microchip website and download 'Filterlab' (it's free).

You fill in a table showing the -3dB points and the order and it does
the rest.

It even draws the schematic for you.
--
Chris Morriss

Preamp low pass filter
 

In article , Chris Morriss
writes
In message , Wally
writes
I have just acquired a slightly beat-up Cambridge A75 preamp (thanks Rob!).
Aside from some tidying up, this will form part of my intended system
upgrade to a bi-amped setup with a low-pass filter on the bass end. I'm
attracted to fitting a second stereo output stage and filter to the preamp -
that would let me try various amps for driving the bass speakers without
having to deal with incorporating the filter into each. Although I can
operate a soldering iron and a multimeter, I'm not terribly clued up on
silicon electronics...

I'm sure there are chips that can take an input from the PCB and drive a
second pair of outputs to the same spec as the existing outputs - but what
ones would be suitable? Those low noise mosfet 741-thingies?

Is there a chip that I can use to construct a filter which doesn't introduce
the sort of phase problems that a speaker-side crossover would? How much
scope is there for being able to tailer the slope of the filter, and make
the slope selectable? Can a subsonic shelf be incorporated? It would be good
to be able to switch the filter off and have the second output provide full
bandwidth.

I'd be grateful for suggestions of other ways to approach the filtering,
suitable chips, maybe some web sites that have circuit / schematics.




If you want an easy way to design and simulate low/high/band-pass
filters of either Butterworth. Bessel or Chebychev with a conventional
op-amp architecture with first to eighth order response, then go to the
Microchip website and download 'Filterlab' (it's free).

You fill in a table showing the -3dB points and the order and it does
the rest.

It even draws the schematic for you.

If you're attempting to put more modern IC's where older ones once went
make sure to run a scope over them to see if their not "hooting" in the
MHz region which they seem to be very good at!....
--
Tony Sayer

Preamp low pass filter
 

Plenty of "Pro Audio" crossover that are perfect for what you ask. Not
expensive, either.

Preamp low pass filter
 

Ian Bell wrote:

I'm sure there are chips that can take an input from the PCB and
drive a second pair of outputs to the same spec as the existing
outputs - but what ones would be suitable? Those low noise mosfet
741-thingies?

NE5532 would be my choice.

Cheers.


Very difficult to design a filter that changes amplitude with
frequency without affecting phase. Laws of physics type stuff.

Oh well - I was hoping there might be some clever chippery that could do
that sort of thing.


How much scope is there for being able to tailer the slope of the
filter, and make the slope selectable?

No problem provided steps of about 6dB/octave are acceptable.

I dare say - I want to be able to match the slope to whatever response I get
out of the upper bass / mid speakers.


Can a subsonic shelf be incorporated?

No problem, in fact hard not to do this.

Bypass is easy too.

Good-o.


--
Wally
www.artbywally.com
www.wally.myby.co.uk/music

Preamp low pass filter
 

Old Fart at Play wrote:

In the analogue world:

You can keep the low-pass and high-pass outputs in phase
with a 4th-order filter. I got a cheap Soundtech active crossover,
which does this, on ebay. It's easy enough to build one though.

I'll have a look for ready-made active crossovers and see how much the cost.


There's some interesting stuff at Siegfried Linkwitz's site:
http://www.linkwitzlab.com

That looks like a good read...


--
Wally
www.artbywally.com
www.wally.myby.co.uk/music

Preamp low pass filter
 

Chris Morriss wrote:

If you want an easy way to design and simulate low/high/band-pass
filters of either Butterworth. Bessel or Chebychev with a conventional
op-amp architecture with first to eighth order response, then go to
the Microchip website and download 'Filterlab' (it's free).

Cheers for that - downloaded.


--
Wally
www.artbywally.com
www.wally.myby.co.uk/music

Preamp low pass filter
 

Old Fart at Play wrote:

Ian Bell wrote:

Very difficult to design a filter that changes amplitude with frequency
without affecting phase. Laws of physics type stuff.


Easy enough to do with a digital signal.

A quick demo would be to low-pass a file with Goldwave or similar,
reverse the file, filter it again and reverse it again.

In the analogue world:

You can keep the low-pass and high-pass outputs in phase
with a 4th-order filter. I got a cheap Soundtech active crossover,
which does this, on ebay. It's easy enough to build one though.

There's some interesting stuff at Siegfried Linkwitz's site:
http://www.linkwitzlab.com



Which include some very nice graphs showing both the frequency and *phase*
response.

Ian

Preamp low pass filter
 

In message , Old Fart at Play
writes
You can keep the low-pass and high-pass outputs in phase
with a 4th-order filter.


Not without an additional all-pass filter you can't. And with an added
all-pass you can make a second order crossover have no phase difference
between the LP and HP sections if you want.
--
Chris Morriss

Preamp low pass filter
 

In article , Chris Morriss
wrote:
In message , Old Fart at Play
writes
You can keep the low-pass and high-pass outputs in phase with a
4th-order filter.


Not without an additional all-pass filter you can't. And with an added
all-pass you can make a second order crossover have no phase difference
between the LP and HP sections if you want.

However you may not actually want that. :-)

Personally, for active filtering, I'd tend to prefer using a LPF, then
creating a HPF output by subtracting the LPF output from the input. The
result if you keep the levels matched is a LP and HP pair of signals whose
vector sum always equals the input. Thus the combined result shows no phase
errors due to the filtering.

For the actual filters I tend to lift the basic designs from the Active
Filter Cookbook by Don Lancaster.

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

Preamp low pass filter
 

Chris Morriss wrote:

In message , Old Fart at Play
writes
You can keep the low-pass and high-pass outputs in phase
with a 4th-order filter.

Not without an additional all-pass filter you can't. And with an added
all-pass you can make a second order crossover have no phase difference
between the LP and HP sections if you want.

Perhaps you would like to refer to the Loudspeaker Design Cookbook
which has graphs of amplitude and phase for various filters
including the fourth order Linkwitz-Riley filter.

--
Roger.

Preamp low pass filter
 

Old Fart at Play wrote:
Chris Morriss wrote:

In message , Old Fart at Play
writes
You can keep the low-pass and high-pass outputs in phase
with a 4th-order filter.

Not without an additional all-pass filter you can't. And with an
added all-pass you can make a second order crossover have no phase
difference between the LP and HP sections if you want.

Perhaps you would like to refer to the Loudspeaker Design Cookbook
which has graphs of amplitude and phase for various filters
including the fourth order Linkwitz-Riley filter.

Remember that it's the acoustical 4th order Linkwitz-Riley that has the
so-called zero phase difference, not some speaker implemented with 4th order
crossovers. The acoustical 4th order is implemented with second-order
crossovers. The other second-order filters are the roll-offs of the speakers
themselves.

Preamp low pass filter
 

In message , Old Fart at Play
writes
Chris Morriss wrote:

In message , Old Fart at Play
writes
You can keep the low-pass and high-pass outputs in phase
with a 4th-order filter.

Not without an additional all-pass filter you can't. And with an added
all-pass you can make a second order crossover have no phase difference
between the LP and HP sections if you want.

Perhaps you would like to refer to the Loudspeaker Design Cookbook
which has graphs of amplitude and phase for various filters
including the fourth order Linkwitz-Riley filter.


OK, I've looked at that, and it doesn't support what you say at all. I
think you are getting confused between a crossover that keeps the
outputs of all the sections in phase at all frequencies (which can only
be done with even-ordered networks, and then only in conjunction with
all-pass phase-correction networks) and the family of crossovers that
attempt to sum to a flat frequency and phase response, even though the
individual outputs have phase differences between them.

(By the way, you can't get a passive filter to have LP and HP outputs in
phase with each other at all frequencies, as it's not possible to
produce the right sort of all-pass network with passive components.)
You can of course make passive networks that have a summed flat
magnitude and phase response, but this is a different thing entirely.

Although all-pass phase corrected active crossovers can be made, they
are not universally liked, as the extra group-delay added by the
phase-compensating all-pass networks mean that the total variation in
phase across the whole audio band can be very considerable. (Whether or
not this is audible on music is debatable).

--
Chris Morriss

Preamp low pass filter
 

Jim Lesurf wrote:

Personally, for active filtering, I'd tend to prefer using a LPF, then
creating a HPF output by subtracting the LPF output from the input.
The result if you keep the levels matched is a LP and HP pair of
signals whose vector sum always equals the input. Thus the combined
result shows no phase errors due to the filtering.

That's a neat trick. (Maybe tri-amping isn't such a bad idea...)


For the actual filters I tend to lift the basic designs from the
Active Filter Cookbook by Don Lancaster.

taking notes


--
Wally
www.artbywally.com
www.wally.myby.co.uk/music

Preamp low pass filter
 

In message , Jim Lesurf
writes
However you may not actually want that. :-)

Personally, for active filtering, I'd tend to prefer using a LPF, then
creating a HPF output by subtracting the LPF output from the input. The
result if you keep the levels matched is a LP and HP pair of signals whose
vector sum always equals the input. Thus the combined result shows no phase
errors due to the filtering.

For the actual filters I tend to lift the basic designs from the Active

Oh yes, I quite agree, a complex phase-compensated crossover has only
one advantage: it does help keep down vertical lobing problems. As
Arnie has also said, it does also depend on the inherent amplitude and
phase response of the drivers.

I use constant-voltage subtraction crossovers, but without any phase
compensation they do force one of the outputs to only roll off at 6db
per octave.
--
Chris Morriss

Preamp low pass filter
 

Chris Morriss wrote:

In message , Old Fart at Play
writes
Chris Morriss wrote:

In message , Old Fart at Play
writes
You can keep the low-pass and high-pass outputs in phase
with a 4th-order filter.

Not without an additional all-pass filter you can't. And with an added
all-pass you can make a second order crossover have no phase difference
between the LP and HP sections if you want.

Perhaps you would like to refer to the Loudspeaker Design Cookbook
which has graphs of amplitude and phase for various filters
including the fourth order Linkwitz-Riley filter.


OK, I've looked at that, and it doesn't support what you say at all. I
think you are getting confused between a crossover that keeps the
outputs of all the sections in phase at all frequencies (which can only
be done with even-ordered networks, and then only in conjunction with
all-pass phase-correction networks) and the family of crossovers that
attempt to sum to a flat frequency and phase response, even though the
individual outputs have phase differences between them.

Have the laws of physics changed since my LDC4 was published?

Section 7.21:Combined response of two-way crossovers
"....exhibit a high-pass and low-pass phase relationship which
is in-phase."

Graphs 7.58 and 7.59 show what I mean.

--
Roger.

Preamp low pass filter
 

In message , Old Fart at Play
writes
Chris Morriss wrote:

In message , Old Fart at Play
writes
Chris Morriss wrote:

In message , Old Fart at Play
writes
You can keep the low-pass and high-pass outputs in phase
with a 4th-order filter.

Not without an additional all-pass filter you can't. And with an added
all-pass you can make a second order crossover have no phase difference
between the LP and HP sections if you want.

Perhaps you would like to refer to the Loudspeaker Design Cookbook
which has graphs of amplitude and phase for various filters
including the fourth order Linkwitz-Riley filter.


OK, I've looked at that, and it doesn't support what you say at all. I
think you are getting confused between a crossover that keeps the
outputs of all the sections in phase at all frequencies (which can only
be done with even-ordered networks, and then only in conjunction with
all-pass phase-correction networks) and the family of crossovers that
attempt to sum to a flat frequency and phase response, even though the
individual outputs have phase differences between them.

Have the laws of physics changed since my LDC4 was published?

Section 7.21:Combined response of two-way crossovers
"....exhibit a high-pass and low-pass phase relationship which
is in-phase."

Graphs 7.58 and 7.59 show what I mean.


Ok, I can see where your confusion is coming from. No, the laws of
physics haven't changed, but the LS cookbook doesn't make things clear.
If you read on in the same section, you'll see that it says "the two
sections sum together flat when the level of both filters is down 6dB at
the crossover frequency".

This is the crux of the issue. To get a flat amplitude response from an
even order filter, the crossover frequency should be at the -6dB point,
but to get the two outputs to be in-phase (actually 180 out of phase,
but this is cured by turning the connections round on either the tweeter
or the bass unit), the crossover frequency needs to be at the -3dB
point.

Here's an example. It's for a second order Butterworth. (And remember
that a 4th order L_R is simply two identical 2nd order Butterworths in
series)

If the crossover is at the -3dB point, the phase is at 90 degrees at
that point, and the HP and LP will be consistently 180 degrees out of
phase, BUT the magnitude will sum to have a 3dB hump.

If the crossover is at the 6dB point, the magnitude will sum to be flat,
but as the phase shift at the -6dB point is 110 degrees (rather than 90)
the phase shifts of the two outputs will not track.

In reality a passive crossover is tweaked to give a compromise (and to
allow for the amplitude/phase characteristics of the drivers...if you've
got a competent design team that is.

An active crossover can be made to have perfect phase tracking between
the HP and LP outputs by judicious use of all-pass networks. (Though as
Jim says, that may not be what you want for best fidelity)
--
Chris Morriss

Preamp low pass filter
 

Chris Morriss wrote:

In message , Old Fart at Play
writes
Chris Morriss wrote:

In message , Old Fart at Play
writes
Chris Morriss wrote:

In message , Old Fart at Play
writes
You can keep the low-pass and high-pass outputs in phase
with a 4th-order filter.

Not without an additional all-pass filter you can't. And with an added
all-pass you can make a second order crossover have no phase difference
between the LP and HP sections if you want.

Perhaps you would like to refer to the Loudspeaker Design Cookbook
which has graphs of amplitude and phase for various filters
including the fourth order Linkwitz-Riley filter.


OK, I've looked at that, and it doesn't support what you say at all. I
think you are getting confused between a crossover that keeps the
outputs of all the sections in phase at all frequencies (which can only
be done with even-ordered networks, and then only in conjunction with
all-pass phase-correction networks) and the family of crossovers that
attempt to sum to a flat frequency and phase response, even though the
individual outputs have phase differences between them.

Have the laws of physics changed since my LDC4 was published?

Section 7.21:Combined response of two-way crossovers
"....exhibit a high-pass and low-pass phase relationship which
is in-phase."

Graphs 7.58 and 7.59 show what I mean.


Ok, I can see where your confusion is coming from. No, the laws of
physics haven't changed, but the LS cookbook doesn't make things clear.
If you read on in the same section, you'll see that it says "the two
sections sum together flat when the level of both filters is down 6dB at
the crossover frequency".

This is the crux of the issue. To get a flat amplitude response from an
even order filter, the crossover frequency should be at the -6dB point,
but to get the two outputs to be in-phase (actually 180 out of phase,
but this is cured by turning the connections round on either the tweeter
or the bass unit), the crossover frequency needs to be at the -3dB
point.

Here's an example. It's for a second order Butterworth. (And remember
that a 4th order L_R is simply two identical 2nd order Butterworths in
series)

If the crossover is at the -3dB point, the phase is at 90 degrees at
that point, and the HP and LP will be consistently 180 degrees out of
phase, BUT the magnitude will sum to have a 3dB hump.

If the crossover is at the 6dB point, the magnitude will sum to be flat,
but as the phase shift at the -6dB point is 110 degrees (rather than 90)
the phase shifts of the two outputs will not track.

Chris, you are still confused but nearly there.

As you say, a 4th order L-R is two 2nd order Butterworths.
At the crossover frequency a B2 filter is -3dB and 90 degrees phase
shift.
Therefore the 4LR is -6dB and 180 degrees.
The LP and HP outputs are in phase at all frequencies
and the voltage sum is constant.

HTH,

Roger.

Preamp low pass filter
 

In message , Old Fart at Play
writes

Chris, you are still confused but nearly there.

As you say, a 4th order L-R is two 2nd order Butterworths.
At the crossover frequency a B2 filter is -3dB and 90 degrees phase
shift.
Therefore the 4LR is -6dB and 180 degrees.
The LP and HP outputs are in phase at all frequencies
and the voltage sum is constant.

HTH,

Roger.

You are quite correct, I'll go and hang my head in shame now :-(

For a 4-th order L-R, you do indeed cross over at the -6dB point and the
outputs of the LP and HP sections are always in phase. (And the
magnitude sums to be flat).

(I even went and SPICEd it to check!)
--
Chris Morriss

Preamp low pass filter
 

In article , Chris Morriss
wrote:
In message , Jim Lesurf
writes
However you may not actually want that. :-)

Personally, for active filtering, I'd tend to prefer using a LPF, then
creating a HPF output by subtracting the LPF output from the input. The
result if you keep the levels matched is a LP and HP pair of signals
whose vector sum always equals the input. Thus the combined result
shows no phase errors due to the filtering.

For the actual filters I tend to lift the basic designs from the Active

Oh yes, I quite agree, a complex phase-compensated crossover has only
one advantage: it does help keep down vertical lobing problems.

I would put this slightly differently. The 'lobing' problem arises as a
result of having an 'array' of speakers in operation in the crossover
frequency region. There will always tend to be a frequency region where the
two units are radiating similar powers. If the speakers are not very close
(in wavelength terms) lobing is then inevitable. The phasing in this region
won't prevent lobing, it will just displace the maxima and minima in
angular terms w.r.t. the line through the speakers and the speaker plane.

As Arnie has also said, it does also depend on the inherent amplitude
and phase response of the drivers.

The key point here for me is the phase responses of the two drivers in the
frequency region where they are tending to radiate similar powers.

If they are 'in phase' at this point, then ensuring the vector sum is
unchanged should mean that the 'far field' power sent normal to the line
through the speaker units (i.e. towards the nominal listener) will be
correct.

However the above makes assumptions about what is the case. So, for
example, if the speakers have phase delays that differ when they are
radiating similar amounts, you'd need to change what you are giving them.
We also have to worry about where the listener may be and the room
acoustic.

All of this is another reason why I'm not really a fan of 'dynamic'
speakers. :-)

The advantage of the method I prefer is that it ensures both constant
amplitude sum (for the correct unit phase behaviour) and constant total
power. Does this by ensuring the vector sum gain from the filtering is
frequency independent. However this may not be what a specific speaker
requires.

Above said, for electronic crossovers, I'd tend to do it this way, then add
any required modifiers to 'pre-correct' the split signals before delivering
them to the power amps and units...

I use constant-voltage subtraction crossovers, but without any phase
compensation they do force one of the outputs to only roll off at 6db
per octave.

The advantage of higher orders is they can cut down to size of the region
where we have an (unwanted) array effect. However you can do this using my
approach, and it saves money as you only need one high-order LPF and then
get the HPF that matches it 'for free'. :-)

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

Preamp low pass filter
 

In message , Jim Lesurf
writes
The advantage of higher orders is they can cut down to size of the region
where we have an (unwanted) array effect. However you can do this using my
approach, and it saves money as you only need one high-order LPF and then
get the HPF that matches it 'for free'. :-)

Slainte,

Jim


But even if your HPF (say) is a 4-th order, the LPF you get by
subtraction is still only a first order. The phase shift of the summed
output is zero, and it does sum flat of course, so it still is a good
thing.

(It took me ages to work this out, but it is correct, and a quick SPICE
simulation shows it)

As I said in an earlier post, I do this on a homemade two-way (M-T-M)
that I use. A second order HPF for the tweeter, and the subtractive LPF
for the Bass units. I put the main filter as the HPF to roll-off the
tweeter reasonably quickly, whereas I wasn't too worried about the
low-order roll-of of the subtractive LP output.


--
Chris Morriss

Preamp low pass filter
 

Chris Morriss wrote:

In message , Old Fart at Play
writes

Chris, you are still confused but nearly there.

As you say, a 4th order L-R is two 2nd order Butterworths.
At the crossover frequency a B2 filter is -3dB and 90 degrees phase
shift.
Therefore the 4LR is -6dB and 180 degrees.
The LP and HP outputs are in phase at all frequencies
and the voltage sum is constant.

HTH,

Roger.

You are quite correct, I'll go and hang my head in shame now :-(

For a 4-th order L-R, you do indeed cross over at the -6dB point and the
outputs of the LP and HP sections are always in phase. (And the
magnitude sums to be flat).

Thanks.

Of course it only works if the woofer and tweeter work well
for an octave or so beyond the crossover frequency, otherwise
as Arny said, you have to consider the acoustical output.

--
Roger.

Preamp low pass filter
 

In article , Chris Morriss
wrote:

This is the crux of the issue. To get a flat amplitude response from an
even order filter, the crossover frequency should be at the -6dB point,

Not quite. The only general requirement would be that the vector sum adds
up to the 0dB level. If you allow the components to be in quadrature, for
example, this can occur if they cross at -3dB.

A problem here is that there is often an ambiguity in discussions due to
the coherent effects of the way the radiation pattern is affected by phase,
and the mean (space-averaged) power around a room, and on axis.

Ideally, you'd want to know the directional and amplitude/phase properties
of the speakers, and the acoustics of the room, and the choice of listening
location. Lacking these, you end up with having to choose a set of
assumptions.

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

Preamp low pass filter
 

In article , Chris Morriss
wrote:
In message , Jim Lesurf
writes
The advantage of higher orders is they can cut down to size of the
region where we have an (unwanted) array effect. However you can do
this using my approach, and it saves money as you only need one
high-order LPF and then get the HPF that matches it 'for free'. :-)

Slainte,

Jim


But even if your HPF (say) is a 4-th order, the LPF you get by
subtraction is still only a first order.

I've not done this for a while, but that strikes me as rather odd (apology
for the pun! :-) ) as a general claim. I think you may find it depends upon
the details of the filter shape of the LPF filter, not just the order.

IIRC when I did some filtering like this a while ago for analysis of the
effects of HF on tweeters the HP anf LP sections (done this way) were of
the same sort or roll-off slopes. May be mis-remembering, though...

The phase shift of the summed output is zero, and it does sum flat of
course, so it still is a good thing.

(It took me ages to work this out, but it is correct, and a quick SPICE
simulation shows it)

I think this may depend upon some specific assumptions you may have made.
However I'll be interested to hear what you can report on this.

Consider designing a LPF that is approaching a 'brick wall' with a flat top
near 0dB and a fall-off to, say, -60dB that occurs over a narrow range.

Where the LPF is near 0dB the output from the 'HPF' must be very small.
However as with transit the turnover region it rises to near 0dB. The
narrower the transition region, the steeper the slope of the turnover of
both the HP and LP outputs.

OTOH if you choose a high order filter that has an inband 'droop' whose
size scales up significantly with the order, you will, indeed reduce the
slope of the HP output in the 'droop' region. But what matters here is the
filter shape, not just the order.

I've forgotten the latin for 'taking an absurd example' for the sake of
extreme illustration. However I won't let my lack of decent classical
education deter me from the following... :-)

Imagine building an analogue version of one of the 96th order low pass
filters used for digital. These can have an inband ripple that is very
close to 0dB, yet die the death in the space of about 2kHz. If you were to
apply the above methods to get the HP difference I doubt it would show just
a first order rolloff as you went down into the low frequency range. I
think the response would change very rapidly over the same 2kHz-ish band.

Or am I missing something?

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

Preamp low pass filter
 

In message , Jim Lesurf
writes
In article , Chris Morriss
wrote:
In message , Jim Lesurf
writes
The advantage of higher orders is they can cut down to size of the
region where we have an (unwanted) array effect. However you can do
this using my approach, and it saves money as you only need one
high-order LPF and then get the HPF that matches it 'for free'. :-)

Slainte,

Jim


But even if your HPF (say) is a 4-th order, the LPF you get by
subtraction is still only a first order.

I've not done this for a while, but that strikes me as rather odd (apology
for the pun! :-) ) as a general claim. I think you may find it depends upon
the details of the filter shape of the LPF filter, not just the order.

IIRC when I did some filtering like this a while ago for analysis of the
effects of HF on tweeters the HP anf LP sections (done this way) were of
the same sort or roll-off slopes. May be mis-remembering, though...

The phase shift of the summed output is zero, and it does sum flat of
course, so it still is a good thing.

(It took me ages to work this out, but it is correct, and a quick SPICE
simulation shows it)

I think this may depend upon some specific assumptions you may have made.
However I'll be interested to hear what you can report on this.


It struck me as rather odd as well when it was first put to me. Do you
have SPICE available to you? It's easy enough to make an arbitary LP or
HP filter and use an "ideal subtractor" element to produce the other
output. It's easy enough to verify it.

I'll try it with a high order filter, (6-th order or so) and run the
simulation. I guess your email address is valid, so I'll email you the
gain/phase plot of the sim run.

Regards,

--
Chris Morriss

Preamp low pass filter
 

In message , Jim Lesurf
writes
I think this may depend upon some specific assumptions you may have made.
However I'll be interested to hear what you can report on this.


I've sent you an email with the SPICE results. They are in .wmf format.
The email is 216k long so I hope you allow emails that size through!

--
Chris Morriss

Preamp low pass filter
 

In article , Chris Morriss
wrote:
In message , Jim Lesurf
writes
I think this may depend upon some specific assumptions you may have
made. However I'll be interested to hear what you can report on this.


I've sent you an email with the SPICE results. They are in .wmf format.
The email is 216k long so I hope you allow emails that size through!

The size is no problem. The snag is that I don't use 'doze, so now will
have to translate the WMFs and hope the translations comes out OK.
;-)

Don't send me a different format (yet!), though. I'll have a bash when I
can and reply in detail by email.

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

Preamp low pass filter
 

In article , Chris Morriss
wrote:
In message , Jim Lesurf
writes


I think this may depend upon some specific assumptions you may have
made. However I'll be interested to hear what you can report on this.


It struck me as rather odd as well when it was first put to me. Do you
have SPICE available to you?

I don't use SPICE much. I tend to write my own programs, or analyse things
like filters using the response equations given in the books like the
cookbook I mentioned. I don't think I have a copy of SPICE to hand that is
32-bit 'clean' for the OS/CPU I use. I'll investigate when I get a chance,
though.


I'll try it with a high order filter, (6-th order or so) and run the
simulation. I guess your email address is valid, so I'll email you the
gain/phase plot of the sim run.

The files have arrived OK. :-)

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html